The invention relates to a process for recovering high-purity oxygen by low-temperature separation of air in a rectification system having a high-pressure column and a low-pressure column, comprising introducing feed air into the high-pressure column, withdrawing an oxygen-containing liquid fraction from the high-pressure column and feeding said withdrawn fraction into the low-pressure column and passing gaseous nitrogen from the low-pressure column to a top condenser having a condensing side and an evaporating side in indirect heat exchange with an evaporating liquid in said evaporation side, so as to at least partially condense said gaseous nitrogen.
A process with these steps is known from DE 3528374 A1. In this two-column process, the low-pressure column has a top condenser in which gaseous top nitrogen is condensed and is recycled as reflux to the low-pressure column. This type of reflux production for the low-pressure column permits the withdrawal of a portion of the nitrogen produced in the double column as a pressurized product. The oxygen-concentrated liquid that accumulates as a bottom product in the low-pressure column is directed entirely to the evaporation side of the top condenser of the low-pressure column and is withdrawn as residual gas.